A competitive inhibition radioimmunoassay has been developed for quantitative analysis of the HBsAg-binding polymerized human albumin. Based on this assay, we have shown that specific binding for glutaraldehyde polymerized albumin is at least 1,000 fold stronger than human albumin polymerized by other procedures such as heat, aging and carbodiimide cross-linking. Specific binding site for polymerized albumin is localized on pre-S gene product in HBsAg. Assay procedure for antibody activity to polymerized albumin has been also developed.